1. Field of the Invention
The present invention is generally in the field of electrical circuits. More specifically, the invention is in the field of power amplifiers.
2. Related Art
Wireless communication devices, such as wireless communication system using time-division multiplexing (“TDM”) communication standard, transmit information in power bursts, which are generated by pulsing a power amplifier. These systems require pulse shaping to minimize undesirable switching transients, which can occur when the power amplifier output is abruptly ramped up to a high power level at the beginning of the power burst and abruptly ramped down at the end of the power burst. The shape of the transmit pulse, i.e. the power burst, can be controlled by adjusting the gain of the power amplifier as a function of time, for example.
In a typical conventional approach, a digital-to-analog converter (“DAC”) is used to control the power amplifier gain and construct a desired pulse shape. However, since the DAC control is digital in nature, it, i.e. the DAC control, requires memory space and clocked reference signals, which can generate undesirable harmonic energy that can interference with radio frequency (“RF”) signals in the power amplifier. Also, the DAC generates a stepped output that must be filtered to eliminate undesirable spectrum content, which is caused by the discrete nature of the DAC output waveform.
Additionally, significant differences exist in the gain control characteristics of power amplifiers provided by different vendors as well as from different generations of power amplifiers. Also, since the DAC and power amplifier are typically provided by a different vendor than the power amplifier, the waveform that comes out of the DAC is not necessarily compatible with the waveform required to drive the power amplifier. As a result, the DAC output waveform must be modified to meet the particular requirements of each power amplifier.
Thus, there is a need in the art for an effective pulse shaping circuit for a power amplifier in a wireless communication system, such as a TDM system.